I think a lot of the Subaru and Beatrice stories that circle around magical conflicts tend to focus on the nature of their contract and the unique, potentially volatile magic they each bring to the partnership. Subaru's 'Invisible Providence' is a mysterious authority that constantly strains him, while Beatrice is a former Gospel-holding Great Spirit with a 400-year-deep well of magical knowledge. Their early dynamic in the Sanctuary arc is a perfect example—it's not just about big flashy spells, but a conflict of magical philosophy and purpose. Beatrice is bound by her library and her duty, her magic feeling ancient and deeply lonely. Subaru, with his crude, self-destructive authorities, represents a chaotic and painful new possibility. The conflict is internal and interpersonal, a clash between centuries of magical stagnation and a desperate, reckless new way of using power to protect others.
You often see this play out in fanworks where they're pitted against other magic users, like the Witches Cult or other Spirits. The tension comes from how they combine their wildly different styles. Beatrice might handle the high-level offensive and defensive spells with precision, while Subaru's role is more strategic, using his Return by Death knowledge to direct her and his authorities to create unsettling, unexpected openings. It's a magical conflict built on asymmetry and forced teamwork. The 'Killing the Great Rabbit' scenario from the main story is a great template for fanfiction authors—it's a battle of attrition that perfectly showcases how Beatrice's massive mana reserves and Subaru's stubborn, loop-informed tactics can clash and then coalesce into a single strategy.
Stories that lean into Beatrice teaching Subaru proper magic, or them researching his authorities together, also mine this vein. The conflict shifts from external enemies to the inherent danger and mystery of Subaru's own power. How does Beatrice, a being of high-level yin magic, react to the corrosive, witch-scented nature of his authorities? That's a rich source of magical and emotional conflict. It's less about fireballs and more about the quiet, terrifying moments where Betty realizes just how broken his magic—and by extension, Subaru himself—truly is. Their bond, forged through those shared magical battles and the resulting exhaustion and understanding, is the real heart of those tales. I love reading fics that capture that post-battle weariness in the library, the unspoken communication as they tend to each other's magical backlash.